1. The Field of the Invention
The present invention relates to methods for forming heat sink and semiconductor chip assemblies. More particularly, the present invention relates to methods for attaching heat sinks to semiconductor chips, wherein the heat sinks are aligned with the corresponding semiconductor chips using a detachable heat sink frame.
2. The Relevant Technology
After a semiconductor chip, such as an integrated circuit, has been manufactured, the semiconductor chip is typically subjected to a packaging process in which it is placed in a condition to be shipped to the consumer or to be used in a manufactured product. Chip packaging has a number of purposes, including protecting the semiconductor chip from environmental conditions, improving the ease of handling or transporting the semiconductor chip, allowing the semiconductor chip to be mounted to a printed circuit board or other external circuitry, and improving the thermal properties of the semiconductor chip. As the structures and circuitry contained within semiconductor chip devices continue to become smaller, it becomes increasingly important for the heat generated during operation of the semiconductor chip to be efficiently removed. Otherwise, overheating may occur, which can cause the breakdown or failure of the semiconductor chip.
One common method for improving the thermal properties of a semiconductor chip is the use of a heat sink to provide a conductive path for the elimination of excess heat. Heat sinks function in one or more of at least two ways. First, heat sinks preferably have relatively high thermal conductivity properties in order to efficiently allow heat to flow from the semiconductor chip and into or through the heat sink. Second, heat sinks preferably have relatively high specific heat values, which are a measure of a material""s ability to store thermal energy. Heat sinks further enhance heat removal if they are in intimate contact with the semiconductor chip over a relatively large area and if the heat sink has a relatively large and efficient interface with the outside environment. Heat sinks may be advantageously formed from metals, common examples of which include nickel, copper, palladium, alloys of the foregoing, and the like. More generally, heat sinks may be constructed from substantially any metal or metal alloy and from some suitable non-metals.
Heat sinks may be attached to or combined with semiconductor chips according to various methods. For example, adhesives may be used to directly attach a heat sink to a semiconductor chip. Alternatively, a heat sink and a semiconductor chip may be encapsulated together using common polymeric materials. In this case, the encapsulating polymeric material further serves to protect the semiconductor chip from environmental conditions.
Conventionally, heat sinks have been combined with semiconductor chips by individually manipulating each heat sink and placing it on or near its corresponding semiconductor chip. Frequently, heat sinks are placed in position with a semiconductor chip shortly before or after a molding step in which the semiconductor chip is substantially encapsulated. It has been found, however, that certain problems are associated with applying heat sinks to semiconductor chips. For example, because the width and length dimensions of conventional heat sinks are often no greater than several millimeters, and may be substantially smaller, it is often difficult to reliably and efficiently align heat sinks with their corresponding semiconductor chips, particularly in high throughput manufacturing settings. It is therefore common for heat sinks to be misaligned with semiconductor chips, to become dislodged before the molding process, or to otherwise fail to be properly positioned. When this occurs, the packaged semiconductor chip may be rendered inoperable or the manufacturing process may be disrupted.
In view of the foregoing, it would be an advancement in the art to provide methods for reliably and efficiently aligning and combining heat sinks with corresponding semiconductor chips. Furthermore, there is a need in the art for such methods to be compatible with common molding processes that are used to encapsulate semiconductor chips. Such heat sink application methods would be particularly advantageous if they could be reliably conducted without the heat sinks becoming dislodged or misplaced before, during, or after the molding process is conducted.
The present invention relates to methods for combining heat sinks with semiconductor chips wherein the heat sinks are placed in their desired positions using a detachable heat sink frame that is removed shortly before or after a molding process is conducted on the semiconductor chips. The methods involve the use of a heat sink frame having one or more heat sinks removably attached thereto. The heat sink frame is aligned with a semiconductor chip frame that has one or more corresponding semiconductor chips attached thereto. After the frames are aligned, the heat sink frame is detached from heat sinks, whereby the heat sinks remain assembled with the semiconductor chips.
Preferably, the heat sinks are positioned in a generally linear arrangement within a strip-like frame. The heat sinks may be advantageously attached to the frame at a number of contact points on the periphery thereof. Attachment of the heat sinks in this manner facilitates the detachment of the heat sink frame. In particular, the contact points may be easily broken in order to release the heat sinks.
The heat sink frames of the invention are preferably used in combination with semiconductor chip frames having one or more semiconductor chips arrayed thereon in substantially the same pattern as the heat sinks. For example, the semiconductor chip frame may be a strip on which the semiconductor chips are arrayed in a generally linear fashion. Accordingly, when the heat sink frame with its heat sinks is positioned generally over and in proximity to the semiconductor chip frame with its semiconductor chips, the heat sinks and semiconductor chips may be combined in a one to one relationship in heat sink and chip assemblies.
According to a first embodiment of the invention, the heat sink frame is aligned with the semiconductor chip frame before a molding process has been conducted. The resulting dual frame assembly is then positioned at a molding surface of a molding machine. While the dual frame assembly is at the molding surface, the heat sink frame is detached from the heat sinks, leaving the heat sinks on the corresponding semiconductor chips. A molding process is then conducted on the heat sink and chip assemblies, whereby a molding material substantially encapsulates both the heat sinks and the semiconductor chips.
The resulting chip package assemblies are removed from the molding surface after the molding step. Excess material is then trimmed from the encapsulation material as needed. According to this embodiment of the invention, chip package assemblies are formed in which the heat sinks and the semiconductor chips are encapsulated together. Using the heat sink frame to align the heat sinks with the semiconductor chips has the advantage of accurately positioning the heat sinks and holding them in place until just prior to the molding process.
In a second embodiment of the invention, the heat sink frame and the semiconductor chip frame are aligned prior to the molding step in much the same manner as in the first embodiment. Likewise, the dual frame assembly is positioned at the molding surface. However, according to this embodiment, the heat sink and chip assemblies are encapsulated without prior detachment of the heat sink frame. After the molding step is conducted, the resulting encapsulated assemblies are removed from the molding surface and excess material is trimmed from encapsulation material as needed.
In this embodiment, the heat sink frame may be detached from the heat sinks at any time after the molding process is completed. For example, detachment of the heat sink frame may be conducted while the encapsulated assembly remains positioned on the molding surface. This step may instead be performed during or after removal of the encapsulated assembly from the molding surface. Furthermore, detachment of the heat sink frame may occur before or during the optional step of trimming excess material from the encapsulation material.
Like other embodiments of the invention, this second embodiment has the advantage of allowing the heat sinks to be accurately aligned with the corresponding semiconductor chips. Furthermore, encapsulating the heat sink and chip assemblies with the heat sink frame attached ensures that the heat sinks remain in the desired position during the molding process.
According to a third embodiment of the invention, the semiconductor chip frame with its associated semiconductor chips is placed at the molding surface without the heat sink frame. Next, a molding step is used to substantially encapsulate the semiconductor chips. At some point after the molding step, the heat sink frame is aligned with the encapsulated semiconductor chips. For example, the heat sink frame may be applied immediately after the molding step while the semiconductor chip frame is still on the molding surface. Alternatively, alignment of the heat sink frame may be conducted during or after removal of the encapsulated semiconductor chips from the molding surface.
After the heat sinks have been positioned on or near the encapsulated semiconductor chips, the heat sink frame is detached from the rest of the assembly. As a result, the heat sinks remain positioned over the encapsulation layer formed during the molding process. Adhesion between the heat sinks and the encapsulation layer may be maintained by providing an adhesive layer therebetween. In some cases, however, depending on the molding material used, the heat sinks may be secured in place without the use of an additional adhesive material. Specifically, in some instances the heat sinks can be caused to adhere directly to the encapsulation layer by placing the heat sinks on the encapsulation layer before the molding material has fully cured.
In this embodiment, the heat sink frame is used to accurately and efficiently position the heat sinks over the semiconductor chips. Moreover, the heat sinks are not present during the molding process and therefore are not subject to being dislodged.
According to any of the embodiments of the invention, it is seen that the shortcomings of the prior art are overcome. For example, the heat sink frame provides a structure for allowing a plurality of heat sinks to be simultaneously manipulated. Thus, the need for individually handling the heat sinks is eliminated. The invention allows the heat sinks to be accurately and reliably applied to semiconductor chips without the risk of becoming misaligned or misplaced, as was previously common. Furthermore, the methods of the invention may be adapted for use with the various molding machines and other processing equipment that are used in the art in order to provide an efficient, high throughput, chip packaging process.